Epoxides such as ethylene oxide, propylene oxide, 1,2-butene oxide and the like are useful intermediates for the preparation of a wide variety of products. The oxirane functionality in such compounds is highly reactive and may be ring-opened with any number of nucleophilic reactants. For example, epoxides may be hydrolyzed to yield glycols useful as anti-freeze components or reactive monomers for the preparation of condensation polymers such as polyesters.
Polyether polyols generated by the ring-opening polymerization of epoxides are widely utilized as intermediates in the preparation of polyurethane foams, elastomers, sealants, coatings, and the like. The reaction of epoxides with alcohols provides glycol ethers, which may be used as polar solvents in a number of applications.
Many different methods for the preparation of epoxides have been developed. One such method involves the use of certain titanium silicalite compounds to catalyze olefin oxidation by hydrogen peroxide. This method is described, for example, in Huybrechts et al., J. Mol. Catal. 71, 129(1992), U.S. Pat. Nos. 4,824,976 (Clerici et al.) and 4,833,260 (Neri et al.) European Pat. Pub. Nos. 311,983, 190,609, 315,247 and 315,248, Belgian Pat. Pub. No. 1,001,038, Clerici et al., J. Catal. 129, 159(1991), and Notari, in "Innovation in Zeolite Material Science," Studies in Surface Science and Catalysis, Vol. 37, p. 413 (1988). The titanium silicalite compounds which have heretofore been found to be useful as epoxidation catalysts are synthetic zeolites corresponding to the general chemical formula EQU xTiO.sub.2 (1-x)SiO.sub.2
wherein x must be in the range of from 0.0001 to 0.04. Expressed a different way, it has been thought that the molar ratio of Si:Ti must be no less than 24:1 in order for such substances to function effectively a catalysts in the hydrogen peroxide oxidation of olefins to epoxides. The low concentration of titanium in these materials indicates that they are silicalites in which a limited number of titanium atoms have taken the place of silica in the lattice framework. Thus, the titanium atoms are isolated from each other by long ##STR1## sequences. The prior art teaches that as the proportion of titanium relative to silica in a titanium silicalite is increased, a greater number of titanium atoms in close proximity to other titanium atoms will be present in the lattice framework. Since the epoxidation activity of the titanium silicalite is believed to be due to isolated titanium atoms, whereas the non-selective decomposition of hydrogen peroxide to water and oxygen (i.e., without transfer of oxygen to the olefin) is thought to take place at titanium atoms located in close proximity to each other, the effective use of titanium-rich silicalites as epoxidation catalysts has not heretofore been thought to be feasible. Titanium silicalites containing a relatively high proportion of titanium to silicon would thus have been expected to perform unsatisfactorily in epoxidation reactions since selectivity to epoxide would be significantly lower.
Huybrechts et al. [J. Mol. Catal., 71, 129(1992)], for example, have reported that an attempt to epoxidize 1-octene using hydrogen peroxide and a titanium silicalite containing 4 mole % titanium (Si:Ti=24) gave a total of only 61% selectivity based on hydrogen peroxide to organic oxidation products. The inefficient use of hydrogen peroxide was ascribed to its decomposition to water and oxygen. Higher selectivities were observed using catalysts containing lower levels of titanium. The conversion of 1-octene was also lower than expected from the activity of silicalites containing lower levels of titanium.
The mechanism by which titanium silicalites catalyze the reaction of hydrogen peroxide with organic substrates is not well understood and the outcome of such reactions is highly unpredictable. For example, when an olefin is reacted with hydrogen peroxide in the presence of titanium silicalite, the product obtained may be either epoxide (U.S. Pat. No. 4,833,260), glycol ether (U.S. Pat. No. 4,476,327), or glycol (Example 10 of U.S. Pat. No. 4,410,501).